The present invention relates generally to electrical current sensing and, more particularly, to a method, an apparatus, and a computer program product for detecting excess current flow in a pluggable component.
An electrical component that is configured to be plugged into a live electrical power system will generally require some type of inrush current detection or limiting. In many cases, this is because power supply bypass capacitors in the component will attempt to charge up very rapidly when suddenly attached to a supply voltage, creating a high current flow within the component. For illustrative purposes, the component may represent a card being inserted into a “live”, i.e., electrically powered, system. As a result, various electrical contacts in the component may be damaged due to arcing, or the supply voltage may be pulled down, thereby interrupting operation of any additional circuitry connected to the same power supply. Further, an almost universal requirement for pluggable components, whether pluggable into live power supplies or not, is that they should be fault current limited. In other words, if a fault occurs, either in a particular component or in a load attached to that component, the input current to the component must be limited in order to prevent the possibility of a fire occurring due to excessive power dissipation, and also to prevent overloading of one or more system power supplies used to provide the supply voltage.
Many solutions to the problems posed by excess current limiting have been advocated. A common solution has been to insert a current limiting resistor in series between a supply bus of the power supply and the component. This approach has its drawbacks, however. First, power is dissipated in the current limiting resistor, even during normal operation, thereby causing excess heat dissipation and reduced component efficiency. Second, the current limiting resistor has two contradictory requirements. During normal operation, there should be a low voltage drop across the resistor so as to ensure that the supply bus voltage remains in regulation. Yet in the presence of a fault, there must be a sufficiently high voltage drop across the resistor to permit the fault to be identified, but such a voltage drop will result in high power dissipation. This drawback limits use of this approach to only very low current applications around a few hundred milliamps.
Another solution to inrush current limiting involves the use of a specially configured power pin. The pluggable component is equipped with a plurality of pins for insertion into a connector energized with electrical power from a power supply. The plurality of pins includes a longer power pin that is longer than the remaining pins such that, upon insertion, the longer power pin makes electrical contact with the connector before the remaining pins make electrical contact. The longer power pin is wired in series with a resistor that limits precharge current. Typically, this resistor is in the range of 1 to 2 ohms. The remaining pins include one or more supply pins for supplying electrical power to the pluggable component while the component is operational. During operation of the pluggable component, the power pin does not supply significant current to the component because the supply pins provide a lower impedance path relative to the power pin. Unfortunately, this scheme does not protect the power supply from faults, which may occur in the pluggable component after the component has been plugged into the connector. This approach also does not provide for over current protection during operation of the pluggable component.
More sophisticated techniques for limiting inrush current involve placing a MOSFET in series between the power connector and the pluggable component. The gate of the MOSFET is controlled, for example, by an RC circuit, which turns the MOSFET on slowly during startup, allowing the power supply bypass capacitors in the component to charge slowly. During steady state operation, the MOSFET provides a low voltage drop.
Unfortunately, MOSFET-based current limiters suffer from one or more problems. For example, MOSFET circuits have an inherently high failure rate due to the mean time before failure (MTBF) of such components, whereas one design objective of a power system is to have the lowest failure rate practicable. Design of MOSFET-based current limiter circuits is complicated by the fact that MOSFETs are inefficient in the linear region, which unfortunately, is the operational region of the device during inrush protection. Furthermore, MOSFET-based current limiters typically require relatively large electrical components to prevent the limiter from overheating due to a fault, thereby resulting in a limiter that is too large and bulky for integration into many types of circuits.
Accordingly, it would be desirable to provide a current limiter that limits the inrush surge current of a pluggable component during startup or in the presence of a fault without experiencing the aforementioned problems of prior art current limiters, in a manner that does not result in excessive thermal dissipation or reduced pluggable component efficiency, and in a manner that provides power metering capabilities in addition to protection.